[LargeFormat] A simple Yellow Filter

[Charles Thorsten]

>In the old days before Apo lenses, photographers wanted to take nice
>sharp photos. But, they did not have all the good stuff that we have
>now.
>The Yellow filter --------
>So they would use a yellow filter to make the lens sharp like an Apo
>lens. 
>Does any one know why -- the yellow filter makes the older lens extra
>sharp? 

APO lenses are designed to focus all primary colors
on the same plane, eliminating chromatic aberration
(color fringing).  Older lenses sometimes suffered from
this especially in the shorter wavelengths, the blue
spectrum.  Using a yellow filter eliminates any blue light,
so the rest focuses sharply on the film plane.  It also
eliminates some of the scattered hazy light from a blue
sky, giving a little more definition to an outdoor scene.
Some people refer to it as a "minus blue" filter.
To me it's kind of a convenient all-purpose filter to have
around, even wth modern lenses.  

-Charlie